In prior art automatic focusing apparatuses for video cameras, appropriate sharpness signals are detected from the high frequency components of brightness signals which are contained in the video signals, and movement of a focusing lens is controlled so that those brightness signals become a maximum. In others, the distance to an object is measured by performing projection-reception operations of infrared light or transmission-receiving operations of supersonic signals to the object, and on the basis of those measured results the movement of a focusing lens is controlled.
The former approach to obtain the focused state by detecting the sharpness signal is based on a principle widely known from disclosures such as NHK Research Laboratory Report, Showa 40, Vol. 17, No. 1, pages 21 to 37. As discussed therein, as the lens approaches a focused state, high frequency components in the video signals increase and peak at the focused state. This approach has advantages in comparison with other approaches such as, for example, the supersonic focusing method or the light projection-reception method. Namely, in those apparatuses, sensors are necessary as a distance measuring means. Sensors such as a supersonic transmitter/receiver element or a projection-reception element are used, and since ordinarily a tele-zoom lens is attached to video cameras or other cameras for use as the above-mentioned sensors, it is required that they are highly accurate even for distances greater than several meters. Thus, the cost becomes very expensive, and an accurate distance measurement for greater than 10 m, for example, becomes difficult. By contrast, measuring the focused state using the sharpness signal does not require any sensor as a particular distance measurement means, thereby making it possible to obtain an automatic focusing apparatus which is low-cost, small-sized, and capable of discriminating the focused state regardless of distance. However, such a device has difficulties of two kinds, as will be described below.
In the system in which appropriate sharpness signals corresponding to the sharpness of picture images are detected from high frequency components of the video signal and the movement of a focus adjustment lens is controlled, when an object moves from an arbitrary focused state to another focused state, the variations of the above-mentioned sharpness signals are used to control the movement of the lens. Namely, since the sharpness signals are at a peak when in a focused state, when the sharpness signal in an arbitrary focused state changes, it is judged that the lens goes from a focused position to an out of focused position, and the movement of the lens is started. However, it is known that the high-frequency components in the video signals vary when the contrast of the object, i.e., the brightness distribution state varies Therefore, for example, when in a state wherein an arbitrary focused state is obtained, such as when an object moves simply left or right without changing the distance to the automatic focusing apparatus, it is considered that the contrast of the object is easily changed.
In the case that the contrast of the object varies as described above, notwithstanding that the distance to the object does not change, the movement of the focusing lens is started. Thus, even though contrast variation takes place, the lens position at which the peak of the sharpness signals is obtained does not change its distance and remains at the same position as before the movement. The lens returns to the original position instantly and stops there, hence it is needless to describe in detail that the moving distance and the moving time are very small amounts. However, even though the moving distance and the moving time of the lens are slight, the fact that the moving action of the lens is carried out every time the contrast varies, notwithstanding that the position of the lens is at a focused position, makes the picture image very unsightly. Thus, the fact that the above-mentioned action takes place was a great problem in the prior art which could not be overlooked.
Although there have been examples of automatic focusing apparatuses employing schemes detecting the sharpness of the video signals in the prior art which were put into practical use, the problem described above was avoided by making the moving speed of the lens a high speed so as to essentially solve the phenomenon that the picture image became unsightly. However, for the purpose of utilizing the feature in the above-mentioned method efficiently, it is strongly desired to solve the above-mentioned problem without merely increasing the moving speed of the lens.
The automatic focusing apparatus utilizing the sharpness signal also has a second difficulty, other than the first difficulty described above, which is caused by the variation characteristic of high-frequency components of video signals influenced by the focal depth of the focusing lens rather than by the above-mentioned distance and contrast. For the sharpness signals obtained by adequate processes of the high-frequency components of video signals, it is known that the characteristics for objects at various distances are such as those shown in FIG. 1 when the aperture value of the lens is kept constant and the focused position is taken to be 3 m. In other words, it is known that when the iris number is kept constant, the shorter the focal length is, the deeper the focal depth becomes. Thus, when such a lens has a short focal length, even if the lens is brought out of the focused position by the focusing operation, variation of the sharpness signal is very small.
A conventional automatic focusing apparatus detects the variation in the sharpness signal due to the distance to an object, and movement of the lens is started first, thereafter confirming its moving direction in accordance with the variation characteristic of the sharpness signal associated with this lens movement. In other words, if the sharpness signal is making its variation in the direction toward a peak point, and if it is in the right direction, the above-mentioned movement of the lens continues until a peak point of the sharpness signal is detected. Consequently, if the object presently under shooting is changed from one at a remote distance to one at a distance of 3 m, for example, and this object at the position of 3 m is making a movement producing variations in its contrast without changing its distance, characteristics of the sharpness signals in two lenses with different focal lengths, for example, in lenses of focal lengths of 80 mm and of 20 mm, are as shown in FIGS. 2(A) and (B), respectively. The broken lines in FIGS. 2(A) and (B) show the characteristics of the sharpness signals when an object does not make a movement resulting in contrast variations.
Time point t.sub.1 in FIG. 2 represents a time point at which the above-mentioned switch-over of the object occurs. In other words, the time interval from t.sub.0 to t.sub.1 corresponds to the sharpness signals in a state with a peak value A.sub.1 occurring when the object is focused at a remote distance, and the fall of the sharpness signal at the time point t.sub.1 corresponds to a variation produced by the transition of the lens state from a focused state to an out of focus state caused by movement of the object.
In general, it is known that when the characteristics of the variations of the sharpness signals with respect to variations of the contrast of an object approaches a peak value which might be obtained at a focused state, the amount of variation becomes large, even if the contrast variations are kept constant. However, when a long focal-length lens is used, whose focal depth is shallow, it is clear from FIG. 1 that the variation of the amplitudes of the sharpness signal with respect to variations of distance to an object is large. Accordingly, even when variations in contrast occur without accompanying variations of distance to the object, the variation amplitude becomes smaller than the amplitude caused by the variations in the focused state. Consequently, in the case of a long focal-length lens whose focal depth is shallow, as is shown in FIG. 2(A), although affected by contrast variation, the lens is moved to a state at which a peak value A.sub.2 for objects at a close distance is obtained. If a fall of a width of D as shown in FIG. 2(A) is obtained, the peak P.sub.1 is judged to be the focused state, and the control is performed based on this peak P.sub.1. Thus, control stops at a state where P.sub.1 is obtained again. The setting of the above-mentioned width D should be done properly so as not to follow the contrast variations having no distance movement.
When a short focal length lens whose focal depth is deep is used, as is clear from the characteristic shown in FIG. 1, even if a distance variation at the time point t.sub.1 is large, variation of the sharpness signal is not so large, and only those sharpness signals whose values are close to a peak value at the focused state are obtained. Therefore, when contrast variation takes place, it is conceivable that its variation amplitude, even if its absolute value is small, becomes larger than the variation amplitudes caused by variations in the focused state. Consequently, when a short focal-length lens is used wherein its focal depth is deep, many peaks appear which are caused by either the contrast variations or coming into the focused state. The control is performed as if it were at the focused state at every peak; therefore, it becomes difficult to judge which peak corresponds to a position of an accurate focused state. As a result of this, there has been a problem that stopping the lens at the focused state under any condition became very difficult, and depending on the situation, the lens eventually kept moving.
As is evident from the characteristic of a lens of a deep focal depth, the variation amplitude of the sharpness signal with respect to the distance variations is small, and even if the distance to the object or the lens position varies to some extent, picture images which are almost in the same state as the focused state are obtained. Thus, when the distance to the object varies as described above, it becomes very difficult to detect from the sharpness signals an accurate focused state. Therefore, in a conventional automatic focusing apparatus utilizing the sharpness signal, the deeper the focal depth is, the more difficult the detection of the focused state becomes.
In the prior art, it is also known that when the focal length is constant, as the aperture value becomes larger (i.e., the aperture area becomes smaller), the focal depth of a lens becomes deeper; therefore, when the aperture value is large, as in the above case, there is a fear that an accurate detection of the focused state is not possible.
The other prior art systems wherein the distance information to the object is gained by the projection-rejection operation of infrared light or the transmission and receiving operation of a supersonic signal its now considered. First, when the projection-reception operation of infrared light is employed, if it is desired to get accurate distance information even for objects at a distance of around 10 meters, the distance detection operation is based upon a means of triangulation. Thus, a high precision light reception element is required, and at the same time, the apparatus relating to the position setting of a light projection and reception part requires a high precision structure, thereby introducing a large difficulty of rise in cost. When the transmission and receiving operation of a supersonic signal is employed, on the other hand, although the cost becomes low, the longer the distance to be measured, and the more accurate the distance information desired, the larger the supersonic transmitter/receiver must be. Thus, the apparatus becomes large, and since it is limited in how large it can be, there has been a problem in that the precision of the distance information is worse in comparison with the above-mentioned light projection-reception operation. Accordingly, a compact automatic focusing apparatus which can be installed with an ordinary zoom lens for use in video cameras and other cameras capable of obtaining the focused state with high precision even for objects at a distance of around 10 m is strongly desired.